Japanese Unexamined Patent Application Publication No. 2000-23455 discloses a switching power supply that reduces loss in a rectifying circuit connected to the secondary side of a converter transformer. When the rectifying circuit is configured by only a diode, loss with voltage drop is generated in the diode, resulting in generation of loss in the rectifying circuit. For avoiding this, in Japanese Unexamined Patent Application Publication No. 2000-23455, a metal-oxide-semiconductor field-effect transistor (MOS-FET) is used in the rectifying circuit to reduce the loss that is generated in the diode, thereby reducing the loss in the rectifying circuit.